1. Field of the Invention
This invention relates generally to coating a conductive substrate by means of electrodeposition so as to achieve enhanced corrosion protection and improve cosmetic appeal. More particularly, this invention relates to coating metallic fasteners in bulk quantities using electrodeposition epoxy films.
2. Brief Description of the Prior Art
It is well known in the art that a conductive substrate can be coated with a composition by means of electrodeposition. Coating a substrate can serve to protect it against corrosion and/or embellish its ornamental appearance.
Generally, there are two known types of electrodeposition processees--anodic electrodeposition and cathodic electrodeposition. Both methods are performed by inducing an electrical current within a coating cell containing a coating compound.
Anodic electrodeposition is accomplished by first immersing the part to be coated into an electrolytic solution containing the coating composition. A positive charge is imparted onto the conductive substrate to be coated. The negatively charged ionic species of the coating composition then move through the electrolyte medium via means of the electrophoretic phenomenon so as to coat the substrate. The coating process continues until the coating composition insulates the substrate at which point the coating process stops. Cathodic electrodeposition brings about similar results by reversing the electrical polarity so that the substrate to be coated acts as a cathode and attracts positively charged ions of the coating composition.
There are advantages and disadvantages associated with both methods of electrodeposition, but on balance, cathodic electrodeposition is generally preferred by the industry for most applications Anodic electrodeposition can provide an efficient and uniform coating upon both exposed and non-exposed or hidden surfaces of the substrate and the resultant coatings are far superior as compared to those attained by means of spray and dip coating methods. Over time, however, chemical side effects of the anodic process can destroy the surface of the conductive substrate and cause components thereof to infiltrate the coating material. Such infiltration can reduce corrosion resistance and promote coating discoloration, usually in the form of yellowing. Cathodic electrodeposition generally does not produce as many problems as are associated with anodic electrodeposition. Accordingly, it allows for broader use applications including the use of lighter-colored coatings. Cathodic electrodeposition also provides better gloss and color retention characteristics, as well as superior coating thickness capabilities. Nevertheless, anodic electrodeposition remains the preferred method for certain applications. For example, anodic electrodeposition of an epoxy film provides an ideal primer surface for a top coating of epoxy or acrylic materials or compositions.
U.S. Pat. No. 4,421,620, issued to Kaylo et al. on Dec. 20, 1983 (hereafter referred to as the '620 patent) discloses a process for preparing a corrosion-resistant metallic substrate by first anodically coating the substrate, and then, immediately, reversing the polarity of the coating cell and cathodically-coating the substrate with another layer of composition. The '620 patent, however, teaches that the substrate must remain electroconductive to a certain degree after being anodically-coated (see column 6, line 1). This requirement necessarily prohibits the substrate from becoming totally insulated by means of an anodic electrodeposition process and therefore limits the effectiveness of the anodic electrodeposition film.